Despite covering more miles, the autonomous vehicles had to ask for human help less often.

Autonomous car prototypes generally have a driver behind the wheel so the human can take over if things go wrong. Waymo, the Google division developing the tech company’s self-driving cars, says that’s happening less and less often as its cars become more adept at handling challenging situations.

That came even as the test cars drove significantly farther in California, covering a combined 635,868 miles last year versus 424,311 in 2015. But the number of disengagement incidents fell from 341 to just 124.

“We’ve been able to make dramatic improvements to our technology because we use each of these disengages to teach and refine our car,” Dmitri Dolgov, head of Waymo’s self-driving technology, wrote in a blog post. “For each event we can create hundreds — and sometimes thousands — of related scenarios in simulation, varying the parameters such as the position and speed of other road users in the area.”

Waymo said that the majority of disengagements last year (51 of the 124 incidents) were due to “software discrepancy” issues. “Unwanted maneuver of vehicle” accounted for 30 disengagements, and “Perception discrepancy” accounted for another 20. Waymo also said that the human operator was generally able to assume control from the self-driving car in less than a second; the average time to intervention was 0.9 seconds.